1. Technical Field
The present invention relates to a single stage process for making polyimide reverse osmosis membranes and, more particularly, asymmetric semipermeable membranes fabricated from aromatic polyimide polymers and copolymers.
2. Background Art
Asymmetric semipermeable membranes and their use in reverse osmosis processes have been known for some time. An asymmetric membrane may be defined as an entity composed of an ultra thin dense skin over a thick swollen porous substructure of the same material. Reverse osmosis processes have typically been employed in the production of potable water from saline and brackish water supplies and from waste streams and involve the diffusion of pure water through a membrane from the water feed solution and the concentration and retention of salt molecules and other impurities by the membrane.
The techniques for making asymmetric membranes from cellulose acetate polymers are well known and are disclosed in U.S. Pat. Nos. 3,133,132 and 3,133,137 issued on May 14, 1964 to Loeb et al., 3,497,072 issued Feb. 24, 1970 to Cannon, and 4,026,978 issued May 31, 1977 to Mungle et al. Cellulose acetate membranes find wide application in the purification of neutral feed waters. However, they are subject to degradation at pH extremes and to both chemical and bacterial attack. Polyamides are also employed in the manufacture of asymmetric membranes, such as is disclosed in U.S. Pat. No. 3,567,632 issued to Richter et al. on Mar. 2, 1971. However, these membranes suffer the disadvantage of extreme sensitivity to degradation by trace quantities of chlorine and other oxidizing agents normally present in process feed streams. Recently, as described in U.S. Pat. No. 3,861,303, issued on June 11, 1974 to Wrasidlo, asymmetric poly(N-amido)imide membranes have been prepared. These polymers, however, have been found to be chemically reactive and soluble in a wide variety of solvents, probably due to the presence of active hydrogens on the amido nitrogens. Moreover, these polymers cannot withstand elevated temperatures.
The inherently greater chemical, biological, and thermal stability of polyimides over the membrane materials used in the prior art makes them attractive candidates for use in reverse osmosis membranes. This was recognized by J. K. Beasley of E. I. duPont de Nemours & Company in a paper presented at the December 1977 meeting of IDEA in Tokyo. He mentioned polyimides as one of over three hundred candidate polymer compositions tested but found unsuitable for reverse osmosis membranes.